This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. ESR has been extensively used to study membrane structure and dynamics with the aid of spin-labeled lipid additives. By means of careful line shape analysis, one can obtain detailed information on the ordering and motion of the lipids in the membrane. Also, recent studies on membranes have shown that high-frequency ESR provides improved orientational resolution. It is certainly true that the dynamical structure of lipid membranes is very complex. The lipid molecules are locally ordered and engaged in overall reorientation. In addition, the internal motions of the chain segments around the many C-C bonds leads to complex dynamics. It could be expected that a combined study at a low frequency (9 GHz) and a high frequency (250 GHz) would enable one to distinguish between the overall motion of the lipids and the internal modes of motion affecting the local site to which the spin label is attached. We have shown this in a study on membrane vesicles composed of pure lipid (DPPC) and of lipid cholesterol in a 1:1 molar ratio using the end chain labeled lipid, 16-PC. The 250 GHz spectra represent a "fast time-scale" such that the overall restricted motion of the lipid in the membrane is frozen out, but it is sensitive to the internal dynamics of the end chain. This leads to a clearer characterization of the dynamic structure of the cholesterol-rich liquid-ordered phase as compared to the liquid crystalline phase. Based on this initial work, we have extended the study to include a range of chain-labeled lipids and spin-labeled peptides at a variety of frequencies: 95, 170, and 250 GHz with our new high frequency CW spectrometers. Channel formation in aligned lipid bilayers (see this year[unreadable]s center highlights) is a particularly vivid demonstration of the utility of multi-frequency ESR to unravel structure and dynamics of these complex, heterogeneous systems.